The invention relates to fiberoptic probes. In particular, the invention relates to a fiberoptic probe having a window element of selected thickness and having a beveled surface for directing light towards the sample. The window thickness and angle of the bevel surface are selected so as to result in efficient illumination of the sample at a close working distance; minimizing the reflection that occurs; the high rejection of any specular reflection that occurs; and efficient packaging within the diameter of the probe.
Spectral analysis, and particularly infrared spectroscopy is a known technique which is used to examine samples and monitor online processes. Typically, for an opaque sample, light of a known spectrum is directed at the sample which absorbs and scatters some of the light at various wavelengths. The diffusely reflected portion of the light is collected and analyzed to produce an absorption spectrum which characterizes the sample.
Fiberoptic probes are used to direct light at the sample and to collect the reflected light. The accuracy and sensitivity of the analysis is dependent upon the quality and quantity of the diffusely reflected light collected from the sample. Good collection efficiency results in higher sensitivity. Also, good rejection of any light which has not interacted with the sample, which can degrade measurement sensitivity (noise sources) is desirable.
The intensity of the diffusely reflected light may be improved by increasing the amount of light to the sample. However, it is not always possible to simply increase the light intensity and thereby improve the collection efficiency. Oftentimes, it is more important to accurately position the source light in the field of view of the fiberoptic probe. At the same time, it is important to limit noise sources which can interfere with the reflected light. Such noise sources occur when spurious light which has not interacted with the sample enters the probe. One example of noise is specular reflectance in which some of the source light is reflected into the field of view of the probe. Specular reflectance oftentimes overpowers the return diffuse reflectance signal and thus limits calibration linearity and measurement sensitivity.
Some known probes which are effective to illuminate the sample and collect reflected light with reasonable efficiency are limited in size and do not adapt well to the addition of a window at the distal end of the device. As scale-up of the probe is attempted, collection efficiency decreases and specular reflectance increases. Probes in which the diameter of the collecting fiber is significantly increased to greater than the diameter of the light transmitting fiber tend to have a reduced collection efficiency because light from the illuminating fiber does not effectively illuminate the field of view of the collecting fiber. There tends to be a blind spot in the center of the collecting fiber.
The working distance, is the distance between the probe end and the sample. Collection efficiency is adversely affected as the working distance increases. This is particularly difficult to alleviate with samples which have low reflectance, such as powders, fabrics and the like. Also, powders which have a grain size smaller than the diameter of the fiber oftentimes produce unwanted artifacts which leads to errors in sensor calibration and in measurement accuracy.
Specular reflectance occurs when the illuminating light encounters a surface where the index of refraction changes, for example, at the fiber end face or at a window at the operating end of the probe. The reflected intensity is a function of the angle at which the light encounters the surface. The loss is somewhat dependent upon the angle at which the light encounters the surface. When light reaches the critical angle, incident light is totally internally reflected within the medium. Not only does the occurrence of specular reflectance reduce the total amount of illuminating light available for the sample, the specularly reflected light may enter the collection fiber and thereby reduce or interfere with the sensitivity of the measurement.
It is thus desirable to position the sample illuminating light as close as possible to the probe so that the illuminating light follows a most efficient path. Further, it is desirable to increase the illumination and the spot size of the illuminating light and to locate the same most efficiently within the field of view of the collecting fiber. It is also desirable to not only reduce specular reflectance, but to do so in a way that results in high rejection of any specular reflectance which may occur.